D 50% slower than C++. What I'm doing wrong?

[Somedude]

Le 14/04/2012 21:53, q66 a écrit :
> On Saturday, 14 April 2012 at 19:05:40 UTC, ReneSac wrote:
>> I have this simple binary arithmetic coder in C++ by Mahoney and
>> translated to D by Maffi. I added "notrow", "final" and "pure"  and
>> "GC.disable" where it was possible, but that didn't made much
>> difference. Adding "const" to the Predictor.p() (as in the C++
>> version) gave 3% higher performance. Here the two versions:
>>
>> http://mattmahoney.net/dc/  <-- original zip
>>
>> http://pastebin.com/55x9dT9C  <-- Original C++ version.
>> http://pastebin.com/TYT7XdwX  <-- Modified D translation.
>>
>> The problem is that the D version is 50% slower:
>>
>> test.fpaq0 (16562521 bytes) -> test.bmp (33159254 bytes)
>>
>> Lang| Comp  | Binary size | Time (lower is better)
>> C++  (g++)  -      13kb   -  2.42s  (100%)   -O3 -s
>> D    (DMD)  -     230kb   -  4.46s  (184%)   -O -release -inline
>> D    (GDC)  -    1322kb   -  3.69s  (152%)   -O3 -frelease -s
>>
>>
>> The only diference I could see between the C++ and D versions is that
>> C++ has hints to the compiler about which functions to inline, and I
>> could't find anything similar in D. So I manually inlined the encode
>> and decode functions:
>>
>> http://pastebin.com/N4nuyVMh  - Manual inline
>>
>> D    (DMD)  -     228kb   -  3.70s  (153%)   -O -release -inline
>> D    (GDC)  -    1318kb   -  3.50s  (144%)   -O3 -frelease -s
>>
>> Still, the D version is slower. What makes this speed diference? Is
>> there any way to side-step this?
>>
>> Note that this simple C++ version can be made more than 2 times faster
>> with algoritimical and io optimizations, (ab)using templates, etc. So
>> I'm not asking for generic speed optimizations, but only things that
>> may make the D code "more equal" to the C++ code.
> 
> I wrote a version http://codepad.org/phpLP7cx based on the C++ one.
> 
> My commands used to compile:
> 
> g++46 -O3 -s fpaq0.cpp -o fpaq0cpp
> dmd -O -release -inline -noboundscheck fpaq0.d
> 
> G++ 4.6, dmd 2.059.
> 
> I did 5 tests for each:
> 
> test.fpaq0 (34603008 bytes) -> test.bmp (34610367 bytes)
> 
> The C++ average result was 9.99 seconds (varying from 9.98 to 10.01)
> The D average result was 12.00 seconds (varying from 11.98 to 12.01)
> 
> That means there is 16.8 percent difference in performance that would be
> cleared out by usage of gdc (which I don't have around currently).

The code is nearly identical (there is a slight difference in update(),
where he accesses the array once more than you), but the main difference
I see is the -noboundscheck compilation option on DMD.